[clang] [clang-tools-extra] [Clang][AST] Introduce `ExplicitInstantiationDecl` to preserve source info and fix diagnostic locations (PR #191658)

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

@boomanaiden154 maybe it is worth to test this with Google's internal workloads.

https://github.com/llvm/llvm-project/pull/191658